I think HSE Expert has given you have a pretty good idea on what HSE Engineering is all about. I can give you a brief description of what the other two engineering are all about but mind you that those are not my specializations so it might not be entirely accurate about their job description.

A pipeline engineer basically handles the arrangement of plant equipments and pipeline routing in a chemical plant. If you work in a design company, the process and instruments engineers will tell you roughly how each process equipments (including measuring instruments, control valves and pipelines) should be connected in each section of process plant via Process Flow Diagram and Piping & Instruments Diagrams. Your job is to work out the plot plan (drawings/maps that tell other engineers where each equipments, operator rooms, office are located at the factory sites). You will need to run a few 3D simulation programs (think of it as 3D autocads) on what is the best arrangements (minimise unnecessary pipe bends, splits, elevation, and etc.) to connect/locate each plant equipment on the process site and where you want to locate the utility facility, control rooms, laboratory and office area around this process site. You will also need to run certain simulation/search from database to work out the stress or if a certain piping materials that are suitable for the designed purpose. This includes coming out with a piping class schedule (pipes composed of various material) and determine each of this piping class suitable design temperature at varying operating pressure.

I do not know much about petroleum exploration engineering, but I would have guess their job is quite similar to geophysicists where they analyse the result from sonic detector, process seismic data, test the soil properties on the explored area and do some simulation to determine if a said area contains potential oil or gas well in it. If you like travelling to unexplored corners of the world (including battling rough seas), this job might be for you.